1. Field of the Invention
The present invention relates to an unsaturated alkyd having a number-average molecular weight of at least 5000 and an unsaturated polyester resin obtained therefrom.
2. Description of the Related Art
It is generally known that an unsaturated alkyd obtained by esterifying, with polyhydroxy alcohols, an .alpha., .beta.-unsaturated polybasic acid or an anhydride thereof such as maleic anhydride or fumaric acid, which is used in combination with a saturated polybasic acid, an unsaturated alicyclic polybasic acid or an anhydride thereof is dissolved in a monomer compolymerizable with the alkyd to form an unsaturated polyester resin.
Such unsaturated polyester resins are used for fiber-reinforced plastics (FRP), paints, casting and the like in a very wide range, and the annual output of the polyester resins has exceeded 200,000 tons.
Although unsaturated polyester resins are very important in the industrial field, the number-average molecular weight of the unsaturated alkyd, which is an important property of the unsaturated polyester resins, is at most about 1000 to 2500, as is common in conventional thermosetting resins. It is not an overstatement to say that there is no alkyd having a molecular weight of at least 3000.
The reason for this is that gelation makes it impossible to obtain a number-average molecular weight of the above value or more by the conventional direct esterification method using a polybasic acid or anhydride thereof and a polyhydroxy alcohol.
In some cases, attempts may be made to employ a conventional deglycol reaction, i.e., reaction at a high temperature of 250.degree. C. or more and a highly-reduced pressure of 1 mm Hg or less in the presence of a metal compound catalyst. Although the inventors attempted to synthesize a high-molecular unsaturated alkyd weight under the above conditions, the attempt was a failure because of inevitable gelation even when a large amount of polymerization inhibitors were added.
A thermosetting unsaturated alkyd cannot be synthesized under the general conditions for synthesis of thermoplastic polyesters. It is thus obvious that there is a fundamental difference between both types of polyesters, and there are no cases where synthesis of a high-molecular unsaturated alkyd is achieved by the above method.
For example, in the case of a thermoplastic polyester, the number-average molecular weight and weight-average molecular weight decrease after the optimum time under reaction conditions has passed, as shown in FIGS. 1 and 2. Namely, this indicates that decomposition reaction takes place. In the case of an unsaturated alkyd, the number-average molecular weight becomes constant at a value slightly lower than the highest value, as shown in FIG. 3. However, the weight-average molecular weight of the unsaturated alkyd steadily increases as shown in FIG. 4. This shows that crosslinking of the unsaturated alkyd takes place at the same time as decomposition thereof. It is clear from the above fact that the thermoplastic polyester and unsaturated alkyd are essentially different in reactions from each other.